With rapid development of electronics, communications and computer industries, camcorders, mobile phones or notebook PCs (Personal Computers) are remarkably being developed. And, as a power source for driving these portable electronic communication equipments, the demand for a lithium secondary battery is increasing day by day. In particular, in application of electric vehicles, uninterruptible power supplies, motor tools or artificial satellites, research and development of the lithium secondary battery as an environmentally friendly power source is lively made inside and outside of the country including Japan, Europe and U.S.A.
A lithium cobalt oxide (LiCoO2) was mainly used as a cathode active material of the lithium secondary battery, however currently other layered cathode material is also used, for example a lithium nickel oxide (Li(Ni—Co—Al)O2) or a lithium composite metal oxide (Li(Ni—Co—Mn)O2), and besides low-costly and highly safe spinel-structured lithium manganese oxide (LiMn2O4) and olivine-structured lithium iron phosphate compound (LiFePO4) are watched with interest.
However, a lithium secondary battery using the lithium cobalt oxide, lithium nickel oxide or lithium composite metal oxide has excellent basic battery characteristics, but insufficient safety, especially overcharge characteristics. To improve the insufficiency, various safety apparatuses have been introduced, for example a shut-down function of a separator, an additive of an electrolyte liquid or a safety device such as a protection circuit or PTC (Positive Temperature Coefficient) device, however these apparatuses were designed in such a circumstance that filling degree of a cathode active material is not too high. Therefore, when filing degree of a cathode active material is increased to meet the demand for high capacity, the safety apparatuses tend to operate insufficiently, thereby resulting in deterioration of safety.
And, a lithium secondary battery using the spinel-structured lithium manganese oxide was once applied to mobile phones, however after the battery was faced with a mobile phone market in pursuit of advanced function, its advantages of low cost and high safety were not utilized due to reduction of energy density.
And, the olivine-structured lithium iron phosphate compound has advantages of low cost and high safety, but it has a very low electronic conductivity, which makes it difficult to expect excellent battery characteristics and a low average operating potential, which does not meet the demand for high capacity.
Therefore, various studies have been made to solve the above-mentioned problems, but an effective solution has not been suggested to date.
For example, Japanese Laid-open Patent Publication No. 2001-143705 disclosed a cathode active material, in which a lithium cobalt oxide and a lithium manganese oxide were mixed. However, the prior art simply mixed the lithium manganese oxide with high safety, and thus did not achieve a sufficient safety improving effect.
And, Japanese Laid-open Patent Publication No. 2002-143708 suggested a cathode active material, in which lithium nickel composite oxides having different compositions were formed in two layers, however the prior art applied the cathode active material formed of two-layered lithium nickel composite oxides having different compositions, and thus it is not considered that the prior art improved safety against overcharge sufficiently and fundamentally.
Japanese Laid-open Patent Publication No. 2007-012441 applied a cathode having at least two-layered cathode active material layers for the purpose of improving overcharge characteristics, and suggested an olivine-structured lithium iron phosphate oxide or a spinel-structured lithium manganese oxide as a layer adjacent to a cathode current collector. An improved overcharge characteristics are expected, however thickness of the oxide layer is formed below its average particle diameter, i.e. about several an and the oxide layer does not contain a conductive material or a conductive additive, and thus it is not considered that high current discharge characteristics are sufficient.
Japanese Laid-open Patent Publication No. 2006-318815 disclosed a technique for coating the surface of secondary particles with a lithium salt or a lithium oxide to improve durability of a lithium nickel oxide. However, it is difficult to coat the entire surface of the secondary particles of a cathode active material with a uniform thickness, and thus a durability improving effect is not remarkable, and besides a wet coating process and a drying process are added, thereby reducing productivity considerably.
Japanese Laid-open Patent Publication No. 2006-19229 suggested to coat the surface of secondary particles with a lithium cobalt zirconium oxide to improve a poor safety of a lithium nickel oxide. However, a wet coating process is applied to coat the surface of the secondary particles with the lithium cobalt zirconium oxide, and thus a safety improving effect is remarkable, but productivity is limited.
Therefore, it requires to suggest a cathode active material having an excellent safety as well as excellent battery characteristics and a method of preparing the cathode active material having an excellent productivity.